Question 6

College Answer

The answer required a description of surfactant composition (phospholipids 85%, neutral 
lipids 5%, and proteins 10%). Phospholipid dipalmitoylphosphatidylcholine is the main 
surface active component. It was expected candidates would provide a description of the
arrangement of the phospholipids with the hydrophilic head in the aqueous phase and the 
hydrophobic tail in the airspace of the alveolus. The effects of surfactant required an 
explanation of surface tension and how this affects alveoli. One good way to explain this was describing how La Place’s law would affect alveoli with, and without,surfactant. As the 
alveoli decrease in size, the surfactant molecules are pushed together and exert a greater 
surface tension lowering effect.
Surfactant is also important in the lung elastic recoil and hysteresis and for alveolar stability 
preventing collapse and thereby improving lung compliance and decreasing work of 
breathing. Surfactant also helps oppose the starling forces in the lung and keep fluid from 
being drawn into the alveoli. Candidates often misunderstood La Place’s law and did not 
explain how surfactant decreases surface tension.

Discussion

Structure of surfactant:

• Composition: of the dry mass, 
  • 85-90% is phospholipid
  • 8-10% is protein - mainly SP proteins A B and C, all small (~4-5 kDa )
  • 2-5% is neutral lipid, eg. cholesterol
• These phospholipids tend to form monolayers which are oriented with the hydrophilic heads buried in the water and the hydrophobic tail chains pointing out into the air.

Effects of lung surfactant on surface tension and lung mechanics:

• Surface tension and the Law of Laplace:

  • Surface tension is the force of attraction between liquid molecules at the liquid-gas interface, expressed in Newtons per meter, which tends to minimise surface area.
  • The surface tension of the alveolar fluid, in its tendency to minimise surface area, is a force promoting the collapse of the alveolus.
  • The relationship of this force to sphere size is described by the Law of Laplace.
  • Law of Laplace (P = 2γ/r) states that the pressure difference between the inside and the outside of an elastic sphere ("Laplace pressure") is inversely proportional to the radius.

• Consequences of Laplace's law for alveoli

  • Smaller partially deflated alveoli will have lower compliance and higher Laplace pressure at any given surface tension
  • Increased Laplace pressure upon small alveoli promotes their collapse, as they empty into neighbouring larger alveoli.
  • Alveolar surface tension adds to the pulmonary capillary hydrostatic gradient (i.e. it promotes the ultrafiltration of oedema fluid)

• Effects of surfactant

  • Alveolar surface tension decreases virtually to zero, particularly when alveoli deflate and phospholipid particles are brought closer together
  • When the alveoli are fully inflated, surfactant phospholipid molecules are farther apart, which decreases compliance on lung deflation, i.e. it produces hysteresis
  • Increased lung compliance results from decreased surface tension 
  • Decreased surface tension results in a decreased capillary-alveolar hydrostatic pressure gradient, decreasing ultrafiltration of fluid

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